<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
   html, body{
        width: 100%;
        height: 100%;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    #center{
        background-color: blueviolet;
        width: 100px;
        height: 100px;
    }
</style>
<body>
    <div id="center"></div>
    
</body>
</html>